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In this Nature League Lesson Plan, Brit explores the definitions and connections between metabolism and aging.

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Nature League is a Complexly production
Welcome back to Nature League!

It's a brand new month, and that means it's time for a brand new theme. This month's theme is metabolism and aging, and I'm not getting any younger just sitting here, so let's start! [CHEERY INTRO MUSIC].

Life on Earth is incredible, and each week on Nature League we explore the amazingness of living things. But one of the reasons why living things are so fascinating is because they don't last forever. All life on Earth comes and goes, both on the species level, but also on the level of individuals.

But why do life forms appear just to eventually pass away? And how does that process happen? To get to the bottom of these questions, we need to look at two related concepts: metabolism and aging.

Let's start with metabolism. In every organism, there are countless processes happening every second within cells. Some of these chemical processes create larger molecules, and some of them break molecules down.

There are specific names for these processes. Anabolism, or anabolic pathways, are the process of building bonds and new molecules- I remember this by thinking of anabolic steroids, and how those can make someone beef up. Another example is the formation of proteins, bigger molecules, from amino acids, which are much smaller.

Basically, anabolic pathways use energy to turn smaller molecules into bigger ones. What about the reverse process? That's called catabolism.

Catabolic pathways break larger molecules into smaller ones. In breaking bonds, energy is released. A great example of this how we all rely on is the breakdown of sugars into usable energy.

Our cells are doing that all the time. And where does metabolism come into it? Well, metabolism is the sum of catabolic and anabolic processes.

In other words, metabolism is the whole sum of making and breaking bonds in an organism. Generally, metabolism is a sort of summary of how matter and energy change and flow throughout the processes of life. Which, ya know, is kinda a big deal.

What's cool about metabolic pathways is that they are really similar across different species. That said, how fast a metabolism runs can be very different across species. If you look at the metabolic rate of warm-blooded animals and adjust for mass, smaller animals typically have faster metabolisms.

But it's really hard to compare across species! There are so many requirements for life and energy in different organisms, and how do you even begin to scale for differences between, say, bacteria and plants? This is an ongoing battle of measurements and scaling, and scientists are still trying to figure out how to compare metabolic rates across life on Earth.

However, one thing that seems pretty likely is that metabolism potentially plays a role in how fast a species ages. And that brings us to the process of aging. It's no surprise that scientists have long been interested in why humans get older, and what affects that.

And also unsurprisingly, the leading theories have changed over time as well. One of the first theories put forth was called the rate of living hypothesis. The idea was that all living things had a certain fixed amount of energy, and if you spent that energy more quickly, you'd die more quickly.

Evidence came from observations that large animals, with slow metabolisms, lived longer than small animals with fast metabolisms. This theory hung around for a while, but over time some contradicting information came into the picture. For example, it turns out that birds actually have much faster metabolisms than mammals of the same size, and yet birds live longer.

So, another theory came along to offer an explanation of the connection between metabolism and aging. This one was called the oxidative stress, or free radical hypothesis, and it goes like this:. Aging happens because over time, cells are damaged by free radicals released as byproducts of metabolic processes.

Specifically, these free radicals are destructive versions of oxygen atoms that come from oxidative metabolism, and the damage accumulates with age. But much like the rate of living hypothesis, the oxidative stress hypothesis of aging also runs into issues with conflicting new data sets and experiments. While some studies have shown that genetic and diet changes lead to increased lifespan and reduced oxidative damage, other studies have failed to demonstrate any effect of antioxidants on longevity.

DNA, proteins, and other macromolecules are for sure affected by free radicals, and an interesting area of open research is testing how organisms with different lifespans have adapted ways of defense against this chemical damage. Keep in mind that metabolism can be affected by several things throughout an organism's lifespan, so even if a strong correlation between the variables we've discussed hasn't quite been validated, there are some other potential players in the aging game. Specifically, recent research into the genes that affect our biological clocks have come up with some fascinating results.

In older individuals, the regulated daily cycles of bodily chemicals get thrown off, and become less regular. Some studies are even suggesting that dysfunctional clock genes in younger humans actually contribute to disease states usually found in older people. A review paper published in 2013 considered a bunch of different species and aging processes in order to come up with nine possible hallmarks of aging- specifically, ones having to do with cell and molecular biology.

One of these aging hallmarks is telomere attrition, and I find it one of the most fascinating age-related processes. Overall, organisms accumulate genetic damage over the course of a lifetime. However, one particular region of chromosomes, called the telomere, gets hit way harder than others.

Telomere attrition, or the shortening of this specific chromosomal region, is common to normal aging processes across mammals. Alright, now a metabolism and aging Lesson Plan wouldn't be complete without a dash of immortality chat. After all, one of the most exciting applications of this topic is the search for longer life.

So which species on Earth have come the closest? While we've discussed how hard it is to perfectly compare metabolic rates of different species, we can at least compare absolute maximum lifespans of individuals. Single, individual trees have been reported with ages over 5,000 years old, and individual fungi have been estimated at older than 1,500 years old.

If we look at the animal kingdom, the longest living species happen to be some of the simplest ones, at least in terms of evolutionary time. Sponges, our most distant animal relatives, can live for thousands of years. The ocean quahog, a species of clam, can live over 500 years.

And you know I wouldn't pass up an opportunity to bring sharks into the mix. It just so happens that a Greenland shark specimen was reported just a few years ago to be several hundred years old...and still living! What about animals more closely related to humans?

The longest living vertebrates include giant tortoises and bowhead whales. These species have had individuals with estimated ages of over 200 years old. But for true immortality, we have to check in with a few amazing groups of life on Earth.

First, the sponges. While some species are noted for living thousands of years, other species are seemingly ageless. Current thoughts on how are based around stem cells- basically, certain sponges have stem cells that continuously proliferate.

Another group of possibly immortals are the cnidarians- a group that includes hydras and jellyfish With hydras, an individual can reorganize its existing cells into a new body form if damaged. So instead of growing more, a rearrangement, called morphallaxis, can be used to keep on living. Perhaps one of the most famous immortals is the hydrozoan Turritopsis nutricula.

This jellyfish can go back and forth between the adult and juvenile life stages, and could possibly continue this process forever. This is why this species has been named “The Immortal Jellyfish” in some news accounts. The relationships between metabolism and aging are complicated, and new research is constantly adding to the complexity of our ideas.

What's for sure is that one reason life on Earth is so special is because it isn't forever, and the mysteries of immortality are still yet to be figured out. Also, jellyfish are rad. We'll be exploring more about metabolism and aging throughout the month, so make sure to come back next week for a metabolism and aging themed Field Trip!

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